Electroluminescent apparatus and image panel



April 2, 1963 T. B. ToMLlNsoN ELECTROLUMINESCENT APPARATUS AND IMAGE PANEL Filed April 5, 1957 3,034,262 ELECTRGLUMENESCENT APPARATUS AND llt/EASE FANEL Terence Bernard Tomlinson, Harrow, England, assigner to Hae'eltine Research, Enc., Chicago, Iii., a corporation of liiinois Filed Apr. 3, 1957, Ser. No. 650,463 Claims priority, application Great Britain Apr. 9, i956 S Ciaiins. (Ci. 25d-213) This invention relates to electroluminescent apparatus and electroluminescent image panels and, particularly, to electroluminescent image-intensifying apparatus for producing images of increased intensity in response to incident images of lesser intensity.

it has been heretofore proposed to utilize electroluminescent apparatus of the kind comprising a layer of electroluminescent material arranged to be excited to luminescence by the application of an energizing voltage across the layer and a photoconductive layer arranged to control the value of the voltage actually applied across the electroluminescent layer in accordance with the intensity of incident radiation lying within a particular range of wave lengths. In devices of this kind, the electroluminescent layer and the photoconductive Ilayer have been connected in series with each other so that -a reduction in the impedance of the photoconductive layer, due to an increase in the intensity of the incident radiation, has resulted in an increase in the voltage applied across the electroluminescent layer and a consequent increase in the light output from such layer.

Devices of the kind heretofore proposed have been proposed for use, for example, as light ampliers in which case the photoconductive material which is employed is a type which is sensitive to visible radiations. As a result, the visible radiations which are incident on the photoeonductive layer produce -a luminous output of increased intensity from the electroluminescent layer. In addition, devices of this kind have been proposed for converting nonvisible radiation, for example, infrared or ultraviolet radiation into visible radiation by a suitable choice of the type of photoconductive material.

It has further `been proposed to form devices of the kind referred to as a unitary device which may, for example, consist of superimposed layers of photoconductive and electroluminescent material sandwiched between two transparent conductive electrodes between which an energizing voltage is applied during the operation of the device. As before, the lowering of the resistance of the photoconductive layer, due to an increase in lthe intensity of the incident radiation, increases the amplitude of the voltae applied across the electroluminescent layer and produces a greater light output. Devices of this kind may be used, for example, as image intensiiers since a spot of light falling on the photoconductive layer serves to produce a corresponding region of increased brightness in the electroluminescent layer. As a result, variations in the intensity of the incident image radiation over the surface of the photoconductive layer give rise to corresponding variations in the brightness over the surface of the electroltmiinescent layer in accordance with the image to be reproduced.

Devices and apparatus of the type heretofore proposed suffer from the disadvantage that equal changes in the incident radiation do not produce equal changes in the light output over the whole range of operation. In other words, the response characteristics of such devices have, in general, been nonlinear.

Moreover, the light output of the electroluminescent layer in such devices is generally negligible at voltages below a certain minimum value which value depends on the electroluminescent material employed, on the dimensions of the element, and on the frequency of the applied voltage. This gives rise to an apparent threshold for the incident radiation below which a corresponding light output is not obtained. 'In other words, these prior devices have :been relatively insensitive to weak or low level incident radiation and, in fact, exhibit a threshold value below which an incident image lof lesser intensity is not reproduced .at all.

It is an object of the invention, therefore, to provide new and improved electroluminesccnt radiation-reproducing apparatus which avoids one or more of the foregoing limitations.

It is another object of the invention to provide new and improved electroluminescent radiation-reproducing apparatus `having an input-output response characteristic of improved linearity.

It is a further object of the invention to provide new and improved electroluminescent radiation-reproducing apparatus having no apparent threshold for low level incident radiation.

It is an additional object of the invention to provide new .and improved electroluminescent radiation-reproducing apparatus having greater -sensitivity to low level incident radiation.

vIt is yet another object of the invention to provide new and improved electroluminescent radiation-reproducing .apparatus which enables the utilization of radiation feedback from the electrolurninescent to the photoconductive element without impairing the operation of the apparatus.

It is a still further object of the invention to provide a new and improved electroluminescent image-reproducing panel capable of amplifying and converting negative images to positive images and vice versa.

In accordance with the invention, electroluniinescent apparatus comprises an electrolurninescent cell comprising a quantity of electroluminescent materal positioned between a pair of conductive electrodes at least one of which is radiation transparent for enabling the radiant glow of the electroluminescent material to escape vfrom the cell. The apparatus also includes a photoconductive cell comprising a quantity of photoconductive material positioned between a pair of conductive electrodes at least one of which is radiation transparent for enabling incident radiation to vary the impedance of the photoconductive material. The apparatus further includes an electrical impedance and means for supplying an energizing voltage. In addition, the apparatus includes means connecting the two cells in parallel with one another and in series with the impedance and the voltage-supply means, whereby variations in the intensity of the radiation incident on the photoconductive material will cause the intensity of glow of the electroluminescent material to vary in an inverse manner.

'For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

Referring to the drawing:

FIG. 1 is a diagrammatic illustration of an elemental form of electroluminescent apparatus constructed in accordance with the present invention;

FIG. 2 is a graphillustrating the relationship between the reproduced brightness and the magnitude of the energizing voltage of an electroluminescent cell of the kind used in the present invention;

FIG. 3 illustrates a further form of elemental electroluminescent apparatus constructed in accordance with the present invention;

lFIG. 4 illustrates a modied form of the apparatus of FIG. 3;

FIG. 5 is a fragmentary plan view of a portion of an electroluminescent image panel constructed in accordance with the present invention, and

FIG. 6 is a cross-sectional view taken on the section line 66 of FIG. 5.

Elemental Apparatus of FIG. 1

Referring to FIG. 1 of the drawing, there is shown an elemental form of electroluminescent apparatus constructed in accordance with the present invention. This apparatus of FIG. l is referred to as being elemental in nature because, forV the case of a complete image, it is capable of reproducing only a single image element. This is not, however, intended to imply any limitation as to the physical size of the apparatus. Nor is it intended to imply that the apparatus is only useful for reproducing intelligible images because, for example, the apparatus might be used as a radiation detector or amplifier Where no image is involved. Also, for the sake of a concrete illustration of its use, the apparatus of FIG. l, as well as that of the other iigures, shall in general be described for the case where the radiation of interest is visible light radiation. It is to be understood that the apparatus is equally useful Ywith other types of electromagnetic radiation such as X-rays, ultraviolet radiation, or infrared radiation.

Considering now the details of the electroluminescent apparatus of FIG. l, such apparatus csomprises an electroluminescent cell 10 of the form lcomprising a quantity of electroluminescent material 11 positioned between a pair of conductive electrodes 12 and 13. The electroluminescent materal 11 may be, for example, copperactivated zinc sulfide. Also, one of the conductive electrodes, in this case the electrode 12, is preferably made to be radiation transparent for enabling the radiant glow of the elcctroluminescent material 11 to escape from the' cell 10. As is known in the art, suc-h radiation transparent conductive electrodemay consist of a very thin metallic iilm of, for example, aluminum or a iilrn of transparent conductive material such as tin oxide. Instead, such electrode might take the form of a grid mesh structure wherein the sizes of the apertures are arranged so that the electrode is essentially transparent. In order -to indicate that such electrode is radiation transparent, it

`is represented diagrammatically by means of a dashed line. Such conductive electrode may be formed on a sheet of dielectric material 14 which, for the case of vis-V ible light, may take the form of glass or a thin layer of mica. lIn the case of the FIG. 1 apparatus, the other conductive electrode 13 may be composed of a relatively thick layer of metaland need not be radiation transparent.

The apparatus of the present invention also includes a photoconductive cell 15 in the form of a quantity of pliotoconductive material y16 positioned between a pair of conductive electrodes 17 `and 18. The photoconductive ma-terial 16 may be a suitably activated cadmium sulfide. As before, one of the conductive electrodes, in this case the electrode 18, is preferably radiation transe parent for enabling, in this case, the incident radiation I to vary the impedance of the photoconductive material 16. This electrode 18 may take any of the forms previously mentioned in connection with the transparent electrode 12 of the cell 10. Also, the other electrode 17 of cell 15 may again be a relatively thick layer of metal.

The 4apparatus also includes an electrical impedance represented, for example, by a resistor 20 and means for supplying an energizing voltage. The energizing voltage should, at least on the basis of present knowledge, be of a fluctuating nature and, hence, may take the formlof an alternating-current voltage of suitable frequency. Means for supplying such an energizing voltage is represented diagrammatically by an alternating-current voltage generator 21 having a pair of output terminals 22 and 23.

The .apparatus of the present invention further includes means connecting the two cells in parallel with one another and in series with the impedance 2i) and the voltage-supply means 21. Such connecting means includes a connecting wire 24 for yconnecting the two electrodes 13 and 17 to the terminal 23 of lthe voltage-supply means and includes connecting wires 25, 26, and 27 for connecting the other two electrodes, namely the conductive electrodes 12 and 18, to the other terminal 22 of the voltage-supply means. It is essential that the impedance represented in this case by the resistor 20 be connected .in series so that the current flow to both -the photoconductive material 16 and the elcotroluminescent material 11 must pass therethrough. In the case shown, this is accomplished by connecting the two wires 25 and 26 to the side of the resistor 20 farthest removed from the voltage source 21.

Considering now the operation of the electroluminescent apparatus just described, it will be noticed that the electroluminescent cell 10 and the photoconductive cell 15 are connected in parallel with one another and that thisV parallel combina-tion is, in turn, connected to the voltage source 21 by way of the series impedance represented by resistor 2%). As a result, the apparatus operates in the bright condition, that is to say, the condition in which it is highly sensitive to voltage changes, when the intensity of incident radiation I is small and the photoconductive element 16 is most sensitive to changes in the incident radiation I. By employing an applied voltage and series-connected impedance of suitable values, there can be made to occur a more linear change in the light output L in response to changes in the incident radiation I than can be obtained in most known devices of the type heretofore proposed where the photoconductive element and .the electroluminescent element are connected in series with each other.

in operation, with no incident light I falling on the photoconductive layer 16 through the transparent electrode 1S, lthere is a high impedance between the electrodes 17 and 18 so that only a small current Hows in the circuit and a relatively high voltage is applied across the electroluminescentlayer 11 located between the electrodes 12 and 13. The value of the voltage actually appearing across the electroluminescen-t layer 11, of course, depends on the relative impedanees of the series resistor Z0 and the electroluminescent layer 11 at the supply frequency. To this end, the construction of the layers and the impedance and supply voltage values are chosen so that the electroluminescent layer 11 is operating at the maximum brightness required for this condition of no incident illamination.

Then, as gradually increasing amounts of incident light, as represented by the arrows I, are allowed to -fall on the photoconductive layer 16, the resistance of .the photoconductive material decreases. The current through the resistor 20 thereupon increases and the voltage across the electroluminescent layer 11 is reduced, thereby resulting in a decreased light output L therefrom.

Although the changes in the light output L will be in the reverse direction to changes in the incident radiation I, by employing two units of apparatus, each constructed in accordance with FIG. 1 and arranged so that the light L emitted by -the electroluminescent element of the first is directed `onto the photoconductive element of the second, an over-all operation is obtained in which the light output varies in the same sense as the incident radiation.

With a -single unit of apparatus, as shown in FIG. l, a more uniform change in light output L with steady changes in the Iincident light I on the photoconductive layer 16 is obtained. This can be more readily appreciated by reference to the graph `of FIG. 2 whichV shows a curve ing-current voltage applied across such an electroluminescent cell it) at a constant frequency. It can be seen that as the voltage is increased from zero there is an apparent threshold voltage V1 at which `a significant amount of light is first emitted by the cell 1i). The rate of change of brightness with respect to voltage, that is, the incremental slope of the curve of FG. 2, then increases as the voltage is increased but subsequently decreases after a voltage V2 is reached. This continues until the cell l0 is operating near the maximum brightness obtainable at the frequency employed.

For the case of prior art devices of the type previously proposed where the photoconductive layer and the electroluminescent layer are connected in series, the relative impedances are such that without employing a supply voltage of an undesiraoly high value only a relatively small voltage is applied across the electroluminescent layer when the incident illumination on the photoconductive layer is small. This situation continues until the voltage across the electroluminescent layer becomes greater than the eective threshold voltage V1 and, until this threshold voltage is exceeded, no significant amount of light will be emitted bythe electroluminescent layer. Furthermore, in such prior art devices further increases in the amount of incident light result in higher voltages being applied across the electroluminescent layer but it can be seen that changes in the amount of light emitted for uniform changes in the incident light become gradually greater and a substantially linear variation in the light output with uniform changes `in the amount of incident light is not obtained.

With the shunt-connected or parallel-connected apparatus of the present invention, on the other hand, the applied voltage is chosen so that the electroluminescent cell 16 operates in the region of V2 when there is little or no incident illumination on the photoconductive cell. An increase in the incident illumination then causes the operating voltage across the electroluminescent cell l@ t decrease towards V1. As a result of this mode of operation, the threshold effect is substantially eliminated for incident radiation of weak intensity. Also, both the electroluminescent cell l@ and the photoconductive cell are in their most sensitive condition when the incident radiation is small. Furthermore, by using elements of suitable materials and dimensions together with a series impedance of an appropriate value, changes in the incident radiation produce a substantially linear change in the brightness of the electroluminescent cell lit over an appreciable range of values. Also, as mentioned, by employing two units of apparatus with the light emitted by the electroluminescent cell of the rst unit arranged to be incident on the photocond-uctive cell of the second unit, an output which is substantially directly proportional to the incident light can be obtained. By a suitable choice of circuit components, a light-amplification eect may also be obtained.

Another feature of the present invention is that it makes practical the use of radiation feedb ck from the electroluminescent cell to the photoconductive cell of the same unit of apparatus without impairing the operation of the apparatus. As a result, an even more linear change in the light output in response to changes in the incident illumination may be obtained. This, of course, is only applicable where the photoconductive material is sensitive to the same type of radiation as emitted by the electroluminescent material. The amount of feedback may, of course, be controlled by limiting the amount of light from the electroluminescent element which is al- -lowed to fall on the photoconductive element. The light feedback will, of course, vary in the reverse direction to variations in the incident radiation, that is to say, it will constitute negative feedback and, by controlling the amount of feedback, for example, by the use of a screen of a suitable transparency, a variable light output giving t5 a substantial linear response to changes in the input radiation can be obtained. This form of operation may most readily be obtained by using a modined form of construction as will now be explained in connection with lFIG. 3.`

Elemental Apparatus of FIG. 3

Referring now to FIG. 3 of the drawing, there is shown a further form of elemental electroluminescent appar-atus constructed in accordance with the present invention wherein the electroluminescent and photoconductive cells are, so to speak, arranged back-to-back to one another with a single conductive electrode common to each cell. The electroluminescent cell includes a quantity of electroluminescent material 3d positioned between a pair of conductive electrodes 31 and 32. Similarly, the photoconductive cell includes a quantity of photoconductive material 33 positioned between a pair of conductive electrodes 32 and 34. As is apparent, the center conductive electrode 32 is thus an electrode which is common to the two cells. The outer two conductive electrodes 3i and 34 are radiation transparent and may be superimposed on corresponding layers 35 and 36 of a dielectric material such -as glass. The series impedance is, in this case, represented by a condenser 37 and is connected in series with the voltage-supply means 21 and its terminals 22 and 23.

The operation of the apparatus of FIG. 3 is generally the same as that of the FIG. 1 apparatus because of the fact that the electroluminescent and photoconductive cells of FG. 3 are effectively coupled in parallel with one another. There is one important exception, however, in that radiation feedback from the electroluminescent layer 3i) to the photoconductive layer 33` automatically occurs unless some means is included for blocking such feedback. As mentioned, such yfeedback is desirable in that it serves to improve the linearity of the device. Where no such radiation feedback is desired, however, such feedback may be prevented by including a thin layer of opaque material between either the electroluminescent layer Sil and the conductive electrode 32 or between the conductive electrode 32 and the photoconductive layer 33 or both. Another way of achieving the same result would be to make the common conductive electrode 32 of suflcient thickness so as to be ysubstantially opaque to the radiation emitted by the electroluminescent layer 3o. Of course, in most cases the radiation feedback, which is a form of negative feedback, will be desired. The amount of such feedback may be controlled by either suitably selecting the transparency yfactor of the common conductive electrode 32 or by utilizing semitransparent layers in place of the opaque layers just mentioned or by a combination of these two techniques. The use of such negative feedback will, of course, in general, somewhat decrease the operating sensitivity of the apparatus. A suitably high operating sensitivity, that is, light-arnpli'iication factor, may, however, be obtained by the use of a plurality of such devices placed one after the other.

Elemental Apparatus of FIG. 4

Referring now to FIG. 4 of the drawing, there is shown a modified `form of electroluminescent apparatus which is generally similar to that of FIG. 3 except that provision has been made whereby the desired series impedance is built into the device itself thus eliminating the need for an external circuit component. To this end, each of the dielectric layers 35' and 36 may include additional conductive electrodes to andtl positioned adjacent the outer surfaces thereof. These additional electrodes ttl and el, which should be radiation transparent, may be formed, for example, by coating suitable films of transparent conductive material on the outer surfaces of the dielectric layers 35 and 36. These additional conductive electrodes it? and 4l are then coupled to the terminal Z3 of the voltage-supply means. In addition, what was formerly the outer conductive electrodes, namely the electrodes 31 and 34, are connected togethenfor example, by the external connecting wire 42. v

In operation, the capacitance across each of the dielectric layers 3S and 36 serves to form the desired series impedance which is connected in series between the voltage-supply means 21 and 4the parallel-connected'elec- `troluminescent and photoconductive cells. It is essential that the conductive electrodes 311and 34 be connected together as indicated by ythe external connecting 'wire 42 in order that -this built-in capacitance of the dielectric layers 35 land 36 may be common to both the electroluminescent and photoconductive cells. In other words, the capacitances of the dielectric layers 35 and 36 are effectively in parallel with one another and this parallel combination is `in series with the parallel combination formed by 'the electroluminescent and photoconductive cells. If desired, either the conductive electrode 40 or the conductive electrode 41 may be omitted, in which case the series capacitance is correspondingly reduced.

Image Panel of FIGS. and 6 Referring now to FIG. 5 .of the drawing, there is shown a plan view of a fragmentary portion of one form of electroluminescent Vimage panel constructed in accordance with the present invention. FIG. 6 is a crosssectional view taken on the section line `6 6 of FIG. 5. As best seen in the cross-sectional View of FIG. 6, this particular form of electroluminescent image panel cornprises a rst radiation transparent conductive layer 50 Kand ya layer -of dielectric material S1, one side of w-hich is contiguous with one side of the first conductive layer 50. The image panel also includes la conductive elec- -trode 52 of grid form Iadjacent; the other side of the dielectric layer 51, a conductive element 53 located within each of Ythe grid apertures of .the grid electrode 52 but not in contact therewith, and photocond-uctive materia-lV 54 located in :the interstices between the conductive elements 53 and the grid members of the grid electrode S2. The image panel also includes `a layer lof semitransparent material 55 adjacent the other side of the grid electrode 52, a layer of electroluminescent material 56 contiguous with the other side of the semitransparent layer 5S, and a second radiation transparent conductive layer 57 contiguous with the other side of the electroluminescent layer 56. In addition, the image panel may 4include a dielectric support layer 58 upon which may be deposited the mentioned second conductive layerV 57. Similarly, the first conductive layer '50 is preferably deposited upon the dielectric layer 51. The grid electr-ode 52 and conductive elements 53 are most easily fabricated by forming them on the lower surface of :the dielectric layer 51 by means of printed circuit techniques. The transparency factor of the semitransparent layer 55 is chosen in accordance with the amount of radiation feedback desired and, were no such feedback desired,'such layer` 55 may be made completely opaque.

The image panel apparatus of FIGS. 5 and 6 also includes means represented by the voltage generator 21 for supplying energizing voltage and means represented by the connecting wire 59 for connecting the first conductive electrode or layer 50 to one terminal 22 of the voltagesupply means 21. Also, the apparatus includes means represented bythe connecting Wire 60 for connecting .the grid electrode 52 and the conductive electrode 57 to another .terminal 23 of the voltage-supply means 21.

A careful study of FIG. 6y will show that the image panel there shown is `actually comprised of a plurality of discrete electroluminescent cells, a plurality of discrete photoconductive cells, and a plurality of discrete electtical impedances. Each discrete electroluminescent cell is formed by that portion of the electroluininescent layer S6 which is located between one of lthe conductive elements 53 and the corresponding portion of the conductive electrode S7. Similarly, each discrete photoconductive cell is formed by that portion of fthe photocon-` ductive material 54 which is located between an edge of one of the conductive elements 53 and the adjacent member of the grid electrode 52. Thus, each conductive element 53Y forms .an electrode common to both ran electroluminescent cell and la pair of photoconductive cells located ion either edge of such element 53, all three of these cells being connected in parallel by connecting the grid electrode 52 and the conductive electrode 57 together and to the same terminal of the voltage-supply means.

Y Each Yof :the discrete electrical impedances is formed by .the capacitance between one of the conductive elements 53 and the corresponding portion of :the first conductive electrode 50. As `is apparent, each of these electrical impedances, in this case capacitances, is in series with the parallel-connected electroluminescent and photoconductive cells associated with the same conductive element 53.

In operation, therefore, each of the elemental units of the complete image panel behaves in the same manner as the elemental units of apparatus already considered. More particularly, for a given image panel unit, the capacitance between the conductive layer 50 and the discrete conductive element 53 acts in the same way as the condenser 37 of the apparatus shown in FIG. 3 so that in operation an alternating voltage appears across the electroluminescent cell formed by the portion of the electroluminescent layer 56 associated with the same conductive element 53, exciting the electroluminescent material to luminescence. Then, if light is allowed to fall on the photoconductive material 54 also associated with this same conductive element 53, the resistance between the conductive element 53 and the grid electrode 52 in that region falls and the voltage across the adjacent part of the electroluminescent cell is reduced, thereby decreasing the brightness of the corresponding region of the'electroluminescent layer 56. In this Way, a radiation image incident on the photoconductive material 54 will give rise to a corresponding negative image on the electroluminescent layer 56. If a positive image is required, two such image panels may be used in conjunction with each other, one after the other, so that the second image panel serves to convert the negative image formed by the first to a positive image on the second. Each of the image panels will, of course, contribute to the gain or light amplification of the image.

It will be appreciated, of course, that electroluminescent apparatus and image panels constructed in accordance with the present invention are not restricted to cases in which the incident radiation lies within the visible range but may also be used to convert nonvisible radiation such as, for example, X-rays, ultraviolet radiation, or infrared'radiation into visible radiation by use of an appropriate photoconductive material.

While there have been described what are at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed to cover all such changes and modifications as fall Within the true spirit and scope of the invention.

What is claimed is:

l. Electroluminescent apparatus comprising: contiguous regions of different materials positioned adjacent one another in the following order-a first region of conductive material, a region of photo-impedance material, a second region of conductive material, a region of electroluminescent material, and a third region of conductive material; means for supplying an energizing voltage; means connecting the rst and-third conductive regions to one terminal of the voltage-supply means; and means connecting the second conductive region to another terminal of the voltage-supply means; at least one of said connecting means including series impedance means for causing said electrolurninescent material to have a maXimum brightness when no light impinges on said photoimpedance material and lesser brightness inversely proportional to any lig'nt impinging on said photo-impedance material.

2. An electroluminescent image panel comprising: a viirst conductive electrode; a layer of dielectric material one side of which is adjacent the iirst conductive electrode; a second conductive electrode of grid form adjacent the other side of the dielectric layer; a conductive element located withiny each of the grid apertures of the second electrode but not in contact therewith; photo-impedance material located in the interstices between the conductive elements and the grid members of the second electrode; a layer of electroluminescent material adjacent the other side of the second conductive electrode; and a third conductive electrode adjacent the other side of the electrolurninescent layer.

3. An electrolnminescent image panel comprising: a first radiation transparent conductive electrode; a layer of dielectric material one side of which is adjacent the first conductive electrode; a conductive electrode of grid form adjacent the other side of the dielectric layer; a conductive element located within each of the grid apertures of the grid electrode but not in contact therewith; photo-impedance material located in the interstices between the conductive elements and the grid members of the grid electrode; a layer of electroluminescent material adjacent the other side of the grid electrode; and -a second radiation transparent conductive electrode adjacent the other side of the electroluminescent layer.

4. An electroluminescent image panel comprising: a tirst radiation transparent conductive layer; a layer of dielectric material one side of which is adjacent one side of the first conductive layer; a conductive electrode of grid form adjacent the other side of the dielectric layer; a conductive element located within each of the grid apertures of the grid electrode but not in Contact therewith; photo-impedance material located in the interstices between the conductive elements and the grid members of the grid electrode; a layer of electrolurninescent material adjacent the other side of the grid electrode; and a second radiation transparent conductive layer adjacent the other side of the electrolurninescent layer.

5. An electrolurninescent image panel comprising: a iirst conductive electrode; a layer of dielectric material one side of which is adjacent the irst conductive electrode; a second conductive electrode of grid form adjacent the other side of the dielectric layer; a conductive element located within each of the grid apertures of the second electrode but not in contact therewith; photoimpedance material located in the interstices between the conductive elements and the grid members of the second electrode; a layer of semitransparent material adjacent the other side of the second conductive electrode; a layer of electroluminescent material adjacent the other side of the semitransparent layer; and a third conductive electrode adjacent the other side of the electroluminescent layer.

6. An electroluminescent image panel comprising: a iirst conductive electrode; a layer of dielectric material one side of which is adjacent the iirst conductive electrode; a second conductive electrode of grid form adjacent the other side of the dielectric layer; a conductive 6 element located within each of the grid apertures of the second electrode but not in contact therewith; photo-impedance material located in the interstices between the conductive elements and the grid members of the second electrode; a layer of opaque material adjacent the other side of the second conductive electrode; a layer of electroluminescent material adjacent the other side of the opaque layer; and a third conductive electrode adjacent the other side of the electroluminescent layer.

7. An electroluminescent image panel comprising: a rst conductive electrode; a layer of dielectric material one side of which is adjacent the iirst conductive electrode; a second conductive electrode of grid form adjacent the other side ot the dielectric layer; -a conductive element located within each of the grid apertures of the second electrode but not in contact therewith; photo-irnpedance material located in the interstices between the conductive elements and the grid members of the second electrode; a layer of electroluminescent material adjacent the other side of the second conductive electrode; a third conductive electrode adjacent the other side of the electroluminescent layer; means for supplying an energizing voltage; means connecting the first conductive electrode to one terminal of the voltage-supply means; and means connecting the second and third conductive electrodes to another terminal of the voltage-supply means.

8. An electroluminescent image panel comprising: a first radiation transparent conductive layer; a layer of dielectric material one side of which is contiguous with one side of the first conductive layer; a conductive electrode of grid form adjacent the other side of the dielectric layer; a conductive element located within each of the grid apertures of the grid electrode but not in contact therewith; photo-impedance material located in the interstices between the conductive elements and the grid members of the grid electrode; a layer of semitransparent material adjacent the other side of the grid electrode; a layer `of electroluminescent material contiguous with the other side of the semitransparent layer; a second radiation transparent conductive layer contiguous with the other side of the electroluminescent layer; means for supplying an energizing voltage; means connecting the til-st conductive layer to one terminal of the voltage-supply means; and means connecting the grid electrode and the second conductive layer to another terminal of the voltage-supply means.

References Cited in the lile of this patent UNITED STATES PATENTS 2,768,310 Kazan et al. Oct. 23, 1956 2,773,992 Ullery Dec. 11,1956 2,836,766 Halsted May 27, 1958 2,891,469 Nicoll June 16, 1959 2,908,824 Nicoll Oct. 13, 1959 FOREIGN PATENTS 63,466 Norway Apr. 28, 1941 157,101 Australia June 16, 1954 OTHER REFERENCES Loebner: Opto-Electronic Devices `and Networks, Proceedings ofthe LRE., Dec. 1955, pp. 1897 to 1906.

Ballentyne: The Phenomenon of Electroluminescence and Its Application in the Electronics Industry, Marconi Review, 4th Qtr. of 1956, pp. 160 to 175. 

1. ELECTROLUMINESCENT APPARATUS COMPRISING: CONTIGUOUS REGIONS OF DIFFERENT MATERIALS POSITIONED ADJACENT ONE ANOTHER IN THE FOLLOWING ORDER-A FIRST REGION OF CONDUCTIVE MATERIAL, A REGION OF PHOTO-IMPEDANCE MATERIAL, A SECOND REGION OF CONDUCTIVE MATERIAL, A REGION OF ELECTROLUMINESCENT MATERIAL, AND A THIRD REGION OF CONDUCTIVE MATERIAL; MEANS FOR SUPPLYING AN ENERGIZING VOLTAGE; MEANS CONNECTING THE FIRST AND THIRD CONDUCTIVE REGIONS TO ONE TERMINAL OF THE VOLTAGE-SUPPLY MEANS; AND MEANS CONNECTING THE SECOND CONDUCTIVE REGION TO ANOTHER TERMINAL OF THE VOLTAGE-SUPPLY MEANS; AT LEAST ONE OF SAID CONNECTING MEANS INCLUDING SERIES IMPEDANCE MEANS FOR CAUSING SAID ELECTROLUMINESCENT MATERIAL TO HAVE A MAXIMUM BRIGHTNESS WHEN NO LIGHT IMPINGES ON SAID PHOTOIMPEDANCE MATERIAL AND LESSER BRIGHTNESS INVERSELY PROPORTIONAL TO ANY LIGHT IMPINGING ON SAID PHOTO-IMPEDANCE MATERIAL. 